The present invention relates to a phase interference microscope for forming images of bumps on a surface of a sample by employing interference of light, and particularly, to a phase interference microscope whose focusing method to the sample surface is improved.
As a microscope which allows bumps on a sample surface to be observed at high accuracy, a phase interference microscope (V. R. Tychinsky, I. N. Masalov, V. L. Pankov and D. V. Ublinsky: OPTICS Comm., Vol.74, (1989), pp.37-40.) has been well known.
In this phase microscope, laser light is irradiated on a surface of a sample having bumps, and the bumps on the sample surface are measured at high resolving power on the basis of the phase difference between light beams reflected from recessed portions and protruding portions of the sample surface.
That is, the resolving power D of the bumps of the sample can be represented by EQU D=m.lambda./(4.pi.) (1)
where .lambda. represents a wavelength of the irradiated laser light and .pi. represents a phase resolving power.
It can be understood that if the wavelength .lambda. is 630 nm and the phase resolving power m is 5 degrees (0.087 rad), the resolving power D of the bumps is about 4 nm, i.e. very high.
Incidentally, when the sample bumps are measured at the high resolving power, a focal point needs to be exactly applied to the sample. If it is not, reflection signals from points other than the measured points are included and the bumps cannot be exactly measured. Particularly, when an XY bearing resolution is raised by employing the phase and polarization, a desirable directional resolving power cannot be obtained unless focusing cannot be exactly operated.
However, there has been a problem that even if the focal point is applied visibly or a passive or active automatic focusing method, focusing can be executed at accuracy of only about .+-..lambda..